During the operation of internal combustion engines which, for example, can be arranged in motor vehicles, so-called blow gas gets through a leakage between pistons and cylinders into a crankcase of the respective internal combustion engine. To prevent unacceptable overpressure within the crankcase or to prevent emission of blow-by gas into the environment, positive crankcase ventilation systems are used. Usually, such a positive ventilation system connects the crankcase by means of a vent line with a fresh gas tract of an internal combustion engine. Within the fresh gas tract, in particular downstream of a throttle valve, a relative negative pressure exists which allows that blow-by gas is sucked-in from the crankcase. Further, during the operation of the internal combustion engine, oil mist is generated. Thus, exhausted blow-by gas contains oil mist. To reduce the oil consumption of the internal combustion engine or to reduce the pollutant emission of the internal combustion engine, the positive crankcase ventilation system normally has an oil mist separator that serves for separating the oil mist carried along in the blow-by gas sucked out of the crankcase and to convey it to a suitable oil reservoir which, in particular, can involve an oil pan attached at the bottom of the crankcase.
A progressive tightening of pollutant emission regulations requires an improvement of the cleaning effect of such oil mist separators. Additional, partially conflicting requirements must be met such as, e.g., the longest possible maintenance-free service life of the oil mist separator, a pressure loss as small as possible, a design as compact as possible, and a separation efficiency as high as possible.
From EP 0 860 589 B1, an oil mist separator for a crankcase ventilation system is known which has a first separator unit and a second separator unit as well as a bypass valve. The first separator unit comprises at least one filter body that is arranged in a blow-by gas path in such a manner that blow-by gas can flow through it. The second separator unit is arranged in a bypass path which bypasses the first separator unit and which is opened by the bypass valve as soon as a differential pressure at the first separator unit exceeds a predefined value. In the known oil mist separator, the second separator unit is formed by a filter body.
Further oil mist separators which have a bypass path controlled by means of a bypass valve for bypassing their separator unit, but which do not have an additional separator unit arranged within the bypass path are known from DE 10 2006 051 143 A1, from DE 10 2005 038 257 A1, from DE 2004 061 938 B3, from DE 10 2004 055 065 A1, from DE 103 59 523 A1, from DE 102 32 044 A1, from DE 197 29 439 A1, from WO 01/92 690 A1, and from DE 203 02 220 U1.